In a semiconductor device manufacturing process, plasma processes such as etching, sputtering, CVD (chemical vapor deposition) and the like are mainly performed on a substrate to be processed, e.g., a semiconductor wafer (hereinafter, simply referred to as “wafer”). As plasma processing apparatuses for carrying out such plasma processes, capacitively coupled parallel plate plasma processing apparatuses are widely used.
In this kind of plasma processing apparatus, a pair of parallel plate electrodes (an upper electrode and a lower electrode) is disposed in a chamber, and a processing gas is introduced into the chamber. By applying high-frequency electric power to at least one of the electrodes, a high-frequency electric field is formed between the electrodes and a plasma of the processing gas is generated by means of the high-frequency electric field, and then a plasma process is performed on a wafer by using the plasma.
Meanwhile, recently, in the ULSI (ultra large scale integration), the downscaling of design rules has been gradually progressing, and a higher aspect ratio of a hole shape has been required. In consideration of this situation, an attempt has been made to further raise the frequency of the high-frequency electric power applied to the above-described parallel plate electrodes to thereby form a high density plasma while maintaining the plasma in a well-dissociated state. Thus, it is possible to form appropriate plasma under lower pressure conditions, so that it is possible to more appropriately cope with the downscaling of the design rules.
However, in the above-described plasma processing apparatus, since the upper electrode is formed of a conductor or a semiconductor, there are the following disadvantages. When the applied frequency of the high-frequency electric power is raised in order to form the high-density plasma, the inductance on surfaces of the electrode to which the high frequency wave is applied reaches a significant level. Thus, electric field becomes stronger at the center portion of the electrode, and the distribution of the electric field becomes non-uniform in the radial direction. When the electric field distribution becomes non-uniform in this manner, the plasma density becomes non-uniform, which affects the uniformity of the etching and so on.
In this respect, for example, according to the technology disclosed in Japanese Patent Laid-open Application No. 2001-298015, an electrode plate of the upper electrode is provided with a hollow portion having a disk shape at the center portion of a rear side thereof, so that the high-frequency electric power supplied to the upper electrode causes the resonance in the hollow portion and the electric field perpendicular to the electrode plate. Thereby, the electric field formed directly under the hollow portion, i.e. at the center portion of the electrode, is controlled. This technology can reduce the non-uniformity of the electric field at the center portion of the electrode, i.e., directly under the hollow portion, compared to the case in which no hollow portion is provided on the rear side of the electrode plate of the upper electrode.
In this manner, the provision of a one-stage hollow portion at the center portion of the rear side of the electrode plate is allowed to remarkably increase the uniformity of the electric field at the center portion of the electrode. However, there is a limitation in increasing a uniformity of the electric field at the edge portion of the electrode to be the same level as that at the center portion of the electrode. For example, when an attempt is made to further increase the uniformity of the electric field at the edge portion of the electrode by adjusting the dimensions (diameter and height) of the one-stage hollow portion, the uniformity of the electric field at the center portion of the electrode tends to decrease. For this reason, it is difficult to further increase the uniformity of the electric field at the edge portion of the electrode while the uniformity of the electric field at the center portion of the electrode is maintained.